Check out this image size calculator that shows you how to convert pixels into inches and inches to pixels.
Your Etsy banner may measure 10.63"w x 1.4"h in PowerPoint, but now you need to make that slide 8.54"w x 1.6"h (which means your finished slide will be 640w x 120h). I've actually made mine 8.53"w x 1.6"h and it loaded with no issues in Zibbet.